Coating compositions for application to magnet wire and like substrate materials are well known in the art. See, for example, U.S. Pat. No. 2,936,296, issued May 10, 1960, to F. M. Precopio and D. W. Fox for "Polyesters From Terephthalic Acid, Ethylene Glycol and a Higher Polyfunctional Alcohol"; U.S. Pat. No. 3,652,500, issued Mar. 28, 1972, to M. A. Peterson for "Process For Producing Polyamide Coating Materials By Endcapping"; U.S. Pat. No. 3,663,510, issued May 16, 1972, to M. A. Peterson for "Process for Producing Polyamide Coating Materials"; U.S. Pat. No. 3,179,614, issued Apr. 20, 1965, to W. M. Edwards for "Polyamide-Acids, Compositions Thereof, And Process For Their Preparation"; U.S. Pat. No. 3,179,634, issued Apr. 20, 1965, to W. M. Edwards for "Aromatic Polyimides And The Process For Preparing Them "; U.S. Pat. No. 3,426,098, issued Feb. 4, 1969, to J. F. Meyer et al. for "Polyester-Polyimide Wire Enamel"; U.S. Pat. No. 3,297,785, issued Jan. 10, 1967, to N. J. George et al. for "Melamine-Aldehyde Resin Modified Polyester Reaction Products"; and U.S. Pat. No. 2,787,603, issued Apr. 2, 1957, to P. F. Sanders for "Aqueous Coating Compositions And Substrates Coated Therewith". One of the most widely used, highly successful, and effective magnet wire enamel compositions is that produced in accordance with U.S. Pat. No. 2,936,296, under the trademark "Alkanex" by General Electric Company. The Alkanex composition is highly suitable for use in commercial wire tower applications, and is widely considered as setting the standard in the trade for a large segment of magnet wire production. Notwithstanding the wide spread commercial use and acceptance of the Alkanex magnet wire composition, the composition has the economic and ecological disadvantage of requiring the use of organic solvents.
Where organic solvents are utilized for commercial wire coating applications, they are driven off during curing of the coatings and are generally not economically recoverable. Many such solvents are becoming economically, ecologically and environmentally prohibitive, making it increasingly desirable to utilize substantially water based wire enamels.
For water based wire enamels to be commercially feasible, they must not only result in enamel coatings on magnet wire which reflect properties equal to or better than properties obtainable from Alkanex type magnet wire, but also the aqueous based enamel compositions must be suitable for application to magnet wire in existing commercial wire tower equipment and under the specifications, conditions and parameters utilized for the commercial operation of such equipment. Wire tower equipment for the application of enamel coatings to magnet wire is shown in U.S. Pat. No. 3,351,329, issued Nov. 7, 1967, to D. W. Thomas for "Wire Coating Oven Apparatus"; U.S. Pat. No. 3,183,605, issued May 18, 1965, to D. D. Argue et al. for "Apparatus for Coating Metals"; and U.S. Pat. No. 3,183,604, issued May 18, 1965, to J. D. Stauffer for "Apparatus and Process for Removing Solvent from Coatings on Metal." To the extent necessary for a more complete understanding of the present invention, reference should be made to the above wire tower patents, the disclosure of which is herein incorporated by reference.
Criteria for electrical insulating materials, such as magnet wire insulations, slot insulations, insulating varnishes and the like have been established in the art. In order to determine whether the insulation on a magnet wire will withstand the mechanical, chemical, electrical and thermal stresses encountered in winding machines and electrical apparatus, it is customary to apply the resin to a conductor, by a method which will be described hereinafter, and to subject the enameled wire to a series of tests which have been designed to measure the various properties of the enamel on the wire.
Various tests, which will be described in detail later, include the abrasion resistance tests, the 25 percent elongation plus 3X flexibility test, the snap elongation test, the 70-30 solvent resistance test, the 50--50 solvent resistance test, the dielectric strength tests, the flexibility after heat aging test, the heat shock test, the cut-through temperature test, and the high temperature dielectric strength loss test. The enamel on a conductor which will withstand the mechanical, chemical and electrical stresses encountered in magnet wire applications and which is operable at temperature of at least 135.degree. C. for extended periods of time must withstand at least 10 strokes with the average of three tests being not less than 20 in the repeated scrape abrasion resistance test, must withstand 980 "grams to fail" in the unidirectional scrape resistance test, must pass the 25 percent elongation plus 3X flexibility test, must show no surface defects in the snap test, must show no attack on the insulation in either of the solvent resistance tests, must have a dielectric strength of at least 1500 v. per mil twisted pair, must show no surface defects when wound on an 3X mandrel after heat aging for 100 hours at 175.degree. C., must show no defects when a 5X coil is aged for 30 minutes at 155.degree. C. in the heat shock test, and must have a cut-through temperature of at least 215.degree. C. under a 1000 gram load for 18 AWG heavy coated insulated magnet wire on copper conductor. In addition, for the same type of magnet wire with Nylon overcoat the insulated conductor must not show a loss in dielectric strength of more than 2/3 of original dielectric strength or a minimum of 1500 volts per mil twisted pair, must show no surface defects when a 3X coil is aged for 30 minutes at 155.degree. C. in the heat shock test, and must have a cut-through temperature of at least 200.degree. C. under a 1000 gram load.
The abrasion resistance test, flexibility test, and snap test are employed to determine the mechanical properties of a magnet wire. Abrasion resistance is a measure of the amount of abrasion an insulated electrical conductor will withstand before the insulating enamel is worn away from the conductor. Repeated scrape abrasion resistance is measured by rubbing the side of a loaded round steel needle back and forth across the surface of an insulated electrical conductor until the enamel is worn away. The number of strokes required to wear the enamel away is referred to as the number of abrasion resistance strokes. Unidirectional scrape resistance is measured by rubbing the side of a round steel needle across the surface of an insulated electrical conductor under increasing load until the conductor is exposed. The load required to expose the conductor is referred to as the "grams-to-fail" load. For a complete description of the procedure followed in abrasion resistance testing where a needle is rubbed back and forth across the insulated electrical conductor, reference is made to NEMA Standard Section MW 24 and this NEMA Standard is incorporated herein by reference.
The flexibility of the enamel on a magnet wire is determined by stretching the enameled conductor and examining the stretched portion of the wire under a binocular microscope at a magnification of 10 to determine if there are any imperfections on the surface of the enamel. The imperfections which may be noted on the surface of the enamel are a series of parallel surface lines or fissures which are perpendicular to the long axis of the wire. This condition of the enamel film is known as crazing. Another defect which can sometimes be observed is a break in the enamel film in which the two sections of the film are actually physically separated and the opening extends in depth to the exposed conductor. This defect is called a crack. A third defect which may be noted is a mar or blemish in the enamel film.
In the elongation plus flexibility test, an insulated copper electrical conductor having a diameter X is elongated 25 percent and wound about a mandrel having a diameter 3X, 2X, and 1X. In the counterpart insulated aluminum electrical conductor flexibility test, the conductor is elongated 15% and wound on a mandrel having a diameter 3X, 2X and 1X. If examination of the enamel under a magnification of ten shows none of the surface defects noted above at a particular mandrel wind, the insulation on the conductor passes the flexibility test for that particular X.
The snap elongation test measures the ability of the insulation to withstand a sudden stretch to the breaking point of the conductor. The insulation on the conductor must not show any cracks or tubing beyond three test wire diameters on each side of the fracture after the insulated conductor is jerked to the breaking point at the rate of 12 to 16 feet per second (3.66 to 4.88 meters per second).
Solvent resistance tests are conducted to determine whether a magnet wire will satisfactorily withstand the chemical stresses found in electrical applications, i.e., whether the enamel is resistant to the solvents commonly employed in varnishes which may be used as an overcoat for the enameled wires. The solvent resistance test is the determination of the physical appearance of an enameled wire after immersion in a refluxing bath of a specification solution. Two solution systems are used for each sample of wire. Both of these solutions contain a mixture of alcohol and toluene. The alcoholic portion if composed of 100 parts by volume of U.S.P. ethanol and 5 parts by volume of C.P. methanol. One solvent test solution (which is designated as 50-50) consists of equal parts by volume of the above alcohol mixture and of toluene. The second solution (which is designated as 70-30) is 70 parts of the alcohol mixture and 30 parts of toluene.
In the usual operation of the test, about 250 ml. of the solution is placed in a 500 ml. round-bottomed, single-necked flask which is heated by a suitable electrical heating mantle. A reflux condenser is attached to the flask and the solution is maintained at reflux temperature. A sample is formed so that three or more straight lengths of the wire having cut ends can be inserted through the condenser into the boiling solvent. After 5 minutes the wire is removed and examined for blisters, swelling or softening. Any visible change in the surface constitutes a failure. Soft (requiring the thumbnail to remove it) but smooth and adherent enamel is considered to pass the 5 -minute test. The samples are then returned to the solvent for another 5 minutes and re-examined for the same defects. If the enamel shows any blisters or swelling at the end of either the 5 -minute or the 10-minute test in the 70-30 solution (the 70-30 solvent resistance test) the enamel has failed the solvent resistance test. If the enamel shows any blisters or swelling at the end of the 5 -minute test in the 50-50 mixture (the 50-50 solvent resistance test) the enamel has failed this solvent resistance test.
The dielectric strength of the enamel film determines whether the insulation on a magnet wire can withstand the electrical stresses encountered in electrical apparatus. The dielectric strength of an insulating film is the voltage required to pass a finite current through the film. In general, dielectric strength is measured by increasing the potential across the insulating film at a rate of 500 volts per second and taking the root mean square of the voltage at which the finite current flows through the film as the dielectric strength.
The type of specimen employed to measure dielectric strength is a sample made up of two pieces of enameled wire which have been twisted together a specified number of times while held under a specific tension. A potential is then placed across the two conductors and the voltage is increased at the rate of 500 volts per second until a finite current flows through the insulation. The voltage determined by this method is referred to as "dielectric strength, volts (or volts per mil), twisted pair". The number of twists and the tension applied to the twisted wire is determined by the size of the bare conductor. A complete listing of the specifications for various wire sizes are described in the aforementioned NEMA Standard Section MW 24.
In order to determine whether a magnet wire may be employed at high temperatures, it is necessary to measure properties of the enameled conductor at high temperatures. Among the properties which must be measured are the cut-through temperature of the enamel, the flexibility of the enamel after heat aging at an elevated temperature, the heat shock characteristics of the enamel, and the dielectric strength loss of the enamel when heated at high temperature in air. Since it is well known that copper is the most common conductor, all of the thermal tests of magnet wire are conducted on copper magnet wire.
The cut-through temperature of the enamel film is measured to determine whether the insulation on a magnet wire will flow when the wire is raised to an elevated temperature under compressive stress. The cut-through temperature is the temperature at which the enamel film separting two magnet wires, crossed at 90.degree. and supporting a given load on the upper wire, flows sufficiently to establish electrical contact between the two conductors. Since magnet wires in electrical apparatus may be under compression, it is important that the wires be resistant to softening by high temperature so as to prevent short circuits within the apparatus. The tests are conducted by placing two 8 inch lengths of enameled wire perpendicular to each other under a load of 1000 grams at the intersection of the two wires. A potential of 110 volts A.C. is applied to the end of each wire and a circuit which contains a suitable indicator such as a line recorder, a buzzer or neon lamp is established between the ends of the wires. The temperature of the crossed wires and the load is then increased at the rate of 3.degree. per minute until the enamel softens sufficiently so that the bare conductors come into contact with each other and cause the indicator to signal a failure. The temperature at which this circuit is established is measured by a thermocouple extending into a thermowell to a point directly under the crossed wires. The cut-through temperature is taken as the temperature in the thermowell at the moment when the current first flows through the crossed wires. Although this temperature is always somewhat lower than the true temperature of the wires, it gives a fairly accurate measurement of the cut-through temperature range of the enameled wire being tested. Magnet wires designated for operating temperatures of at least 135.degree. C.
When measuring properties of an insulating film such as percent elongation after heat aging, heat shock, weight loss after heating in vacuum, and dielectric strength loss after heating in air, what is actually being measured is the effect of thermal degradation of the enamel on the particular properties being measured. The most straightforward method of measuring this thermal degradation of an enamel on a wire is to maintain the enameled wire at the temperature at which it is desired to operate the wire until decomposition takes place. This method is impractical in the evaluation of new materials, however, because of the relatively long periods of time involved. Thus, it might be found that an enameled wire may operate successfully at a temperature of 135.degree. C., for example, for 5 to 10 years before any substantial thermal degradation takes place. Because it is obviously impractical to wait such a long period of time to find out whether a resin is satisfactory for magnet wire enamel, it is customary to conduct accelerated heat life tests on these enameled wires. Since thermodynamic theories show that the rate of a given reaction can be determined as a function of temperature, it is possible to select elevated temperatures for thermal tests of enamel films and to calculate the thermal properties of the enameled wire at the desired operating temperature from these accelerated test data. Although it might be expected that degradation reactions which occur at elevated test temperature might not occur at temperatures at which the magnet wire is to be operated because of activation energies required to initiate certain reactions, experience has shown that accelerated heat life tests are an accurate method for determining the heat life of a material at operating temperatures.
In determining whether an enamel film will lose its flexibility after extended periods of time at operating temperature, it is customary to heat age a sample of the enameled wire. In practice it has been found that for a magnet wire to be satisfactory for use in dynamoelectric machines at temperatures of at least 135.degree. C. a sample of the enameled wire having a conductor diameter X must show no surface defects when wound on a mandrel having a diameter of 3X after heat aging for 100 hours in a circulating air oven maintained at a temperature of 175.degree. C. It should be appreciated, however, that the value of the heat age test is questionable when the insulated wire includes a Nylon overcoat, and a failure under said circumstances is not indicative of a defect in the base insulation.
The effect of high temperatures on the flexibility of a magnet wire enamel may also be measured by winding a sample of the enameled wire having a conductor diameter X on a mandrel having a diameter of 5X, removing the sample of wire from the mandrel and placing it in a circulating air oven maintained at 155.degree. C. After 30 minutes the sample of wire should show no surface defects in any of the windings in order for the enameled wire to have sufficient flexibility for steady operation at at least 135.degree. C. This test is known as the heat shock test.
The final thermal requirement of a magnet wire which is to be used at elevated temperatures is that the dielectric strength of the enamel film remains sufficiently high at elevated temperatures after a long period of operation so that no short circuits occur between adjacent magnet wires. For a magnet wire to be satisfactory for operation at a temperature of at least 135.degree. C. its dielectric strength should not be less than two-thirds of the initial dielectric strength after being maintained at a temperature of 175.degree. C. for 100 hours in an oven circulating air having a relative humidity of 25 percent at room temperature. This change in dielectric strength is measured as the dielectric strength, volts (or volts per mil) twisted pairs, both before and after the 175.degree. C. heat aging.
The conditions for commercial wire tower operation depend principally upon the type and diameter of the magnet wire being coated, as well as on the characteristics of the coating composition itself. Briefly, for commercially economical operation, the wire tower must apply a wet enamel coating of between about 2.0 and about 3.5 mils on the diameter to magnet wire ranging in size from a diameter of about 0.0022 inch to a diameter of about 0.144 inch. Both copper and aluminum are commonly used magnet wire materials, and it will be appreciated that the wire tower conditions will vary depending upon which material is utilized as a result of the difference in their thermal conductivity. In particular, for aluminum wire the wire speed in the tower will range from 80 to 125 feet per minute for 0.0126 inch diameter wire to 25 to 28 feet per minute for 0.0605 inch diameter wire. For copper wire, the economical tower speeds are 125 feet per minute for 0.0022 inch wire to 16-20 feet per minute for 0.144 inch wire. The temperature zones and ranges in the wire tower must be sufficient to effectively drive off the solvent, principally water, and heat cure the coating. To this end, the wire tower temperatures are set in the bottom, or solvent removal zone, at a level which will remove the solvent without causing bubbles or blisters, and in the top or cure zone at a level which will effect the desired polymerization and cure without damaging the enamel. Those skilled in the operation of wire towers will be readily able to establish the optimum temperature for operation of the tower. It will be apparent to those skilled in the art that the operation of a wire tower must be such as to produce coated wire at a cost which is commercially competitive under the prevailing market conditions. Thus, the ability to coat and cure magnet wire at high speeds and under temperature conditions which will not adversely affect characteristics and properties of the base wire substrate is of substantial commercial importance. Also of importance is the conservation of valuable organic solvents as well as conservation of the energy required to either recover or dispose of such solvents with a minimum effect on the surrounding community environment.
Illustrative aqueous based wire enamels are disclosed in detail in the co-pending application of Marvin A. Peterson, Ser. No. 501,932, filed Aug. 30, 1974, for "Aqueous Polyester Coating Composition." For a detailed discussion of such compositions, reference should be made to that application, and the disclosure of that application is incorporated herein by reference.